N,N-Diarylanilinosquaraines and Their Application to Organic Photovoltaics

Abstract

We report new derivatives of symmetric squaraine dyes with N,N-diarylanilino substituents that have high solubility and high absorptivity (ε = 0.71–4.1 × 105 M–1 cm–1) in the red solar spectral region (λmax = 645–694 nm) making them promising candidates for application in organic photovoltaics (OPVs). Unsymmetrical N,N-diisobutylanilino- and N,N-diphenylanilino(diphenylamino)squaraines have also been prepared that give blue-shifted absorption spectra (λmax = 529–535 nm) relative to their symmetric counterparts. Compared to bis(N,N-diisobutylanilino)squaraine, both symmetrical and unsymmetrical N,N-diarylanilino squaraines show markedly broader absorption bands in solution than their N,N-dialkylanilino squaraine counterparts: the full width at half-maximum (fwhm) for N,N-diarylanilino squaraines range from 1280–1980 cm–1, while the fwhm value for the N,N-diisobutylanilino squarine is only 630 cm–1. The absorption bands for thin films of N,N-diarylanilino squaraines broaden further to 2500–3300 cm–1. N,N-Diarylanilino squaraines are fluorescent, albeit with lower quantum yields than bis(N,N-diisobutylanilino)squaraine (ϕPL = 0.02–0.66 and 0.80, respectively). OPVs were prepared with solution processed squaraine layers using the following structure: ITO/squaraine (66–85 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å), BCP = bathocuproine. Devices using thin films of the bis(N,N-diarylanilino)squaraines as donor layers show improved performance relative to OPVs prepared with bis(N,N-dialkylanilino)squaraines, i.e. bis(N,N-diisobutylanilino)squaraine: open-circuit voltage V oc = 0.59 ± 0.05 V, short-circuit current J sc = 5.58 ± 0.16 mA/cm2, fill factor FF = 0.56 ± 0.03, and power conversion efficiency η = 1.8 ± 0.2% under 1 sun, AM1.5G simulated illumination, compared with bis(N,N-diphenylanilino)squaraine: V oc = 0.82 ± 0.02 V, J sc = 6.71 ± 0.10 mA/cm2, FF = 0.59 ± 0.01, and η = 3.2 ± 0.1%. Morphological studies of thin films suggest that the solubility of bis(N,N-diarylanilino)squaraines plays an important role in controlling the optoelectronic properties of the OPVs.

Structure and Disorder in Squaraine–C60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor–Acceptor Interface

“…Partial charges for DPSQ were determined by a fi t of the electrostatic potential obtained from a density functional theory (DFT)-based geometry optimization at the B3LYP/6-31G(d,p) level of theory with Gaussian 09 (Revision B.01). [ 51 ] To develop DPSQ slab models for the interface studies, the crystal structure of Wang and co-workers [ 22 ] was used to construct 5 × 5 × 5 (5.6 nm × 5.9 nm × 6 nm, 19 000 atoms) super-cells of the DPSQ (001) and (1-11) lattice (Figure 2 ). The super-cell periodicity was maintained in the xy plane while a 20-nm void space was created in the z-direction to create an isolated slab.…”

“…As a fi rst step, we examine two slab confi gurations that correspond to fi lms developed through differing processing procedures described previously: [ 23 ] an ordered confi guration (O) to represent more crystalline fi lms, and a disordered confi guration (D) to represent the as-cast, amorphous DPSQ fi lm: i) Ordered: Both thermal and solvent-vapor annealing procedures prior to C 60 deposition (denoted as 'Pre-C 60 ') lead experimentally to an ordered, crystalline DPSQ fi lm with a molecular d-spacing of 15 Å. [ 22 ] Notably, this d-spacing differs from that found in solvent-grown crystals of DPSQ [ 22 ] and is shorter than the full length of an isolated DPSQ molecule (≈19 Å) determined from the molecular geometry in the solvent-grown crystal. Here, we use two crystal facets-the (001) and (1-11) surfaces (see Figure 2 )-to represent ordered structures as the exact nature and orientation of the ordered DPSQ fi lm are not experimentally known.…”

“…[ 18,[21][22][23][24][25][26][27] For instance, Thompson, Forrest, and co-workers have exploited 2,4-bis[4-( N,N -diphenylamino)-2,6-dihydroxyphenyl] squaraine, DPSQ, see Figure 1 , as it has good absorption in the red portion of the visible spectrum (>650 nm), while thermal and solvent-vapor annealing (SVA) techniques can be used to manipulate the relative crystallinity/order of DPSQ-C 60 active layers. [ 21,23,28 ] In particular, fi ne control of the bulk and interfacial morphologies of DPSQ-C 60 bilayers lead to a 30% increase in PCE (3.4% to 4.8%).…”

A Series of Squaraine Dyes: Effects of Side Chain and the Number of Hydroxyl Groups on Material Properties and Photovoltaic Performance

“…Notably, the results obtained for SQ1 and SQ2 are consistent with previously reported results. 30,31 The details of X-ray crystallography results for SQ2, SQ3, and SQ4 can be viewed from the crystallographic information files (CIF files, shown in the Supporting Information). SQ1, SQ3, and SQ4 have the same monoclinic crystal system with different space groups of P2(1)/n, P2 1 /c, and P2 1 /c, respectively.…”

Merocyanines for vacuum-deposited small-molecule organic solar cells

“…These reports clearly indicate that other factors must be considered in order to improve merocyanine-based device performance. Interestingly, a recent work from Thompson reported a boosted up PCE of 5.70% with a squaraine-centered D-A-D donor, where the hole mobility was improved by the incorporation of diarylamino groups typically found in hole-transporting materials used in OLEDs [36]. Along this line, we synthesized three merocyanine dyes (DPPT [35], DTPT, and 1-NPPT, Scheme 1) based on thiazolidenemalononitrile as the electron-poor block while three diarylamino phenylenes are the electron-rich blocks.…”